The present invention relates to a control system for an engine having an intake air passage and, more particularly, to a control system having a device which measures intake airflow and which can effectively detect an operating state of the engine and can electronically control the air/fuel ratio of the engine.
To electronically control an engine, the engine operating state is monitored, and a signal corresponding to the monitored operating state is generated. A suitable fuel injection quantity and a proper ignition timing are calculated in accordance with this signal. Fuel injection control and ignition timing control are performed in accordance with the calculated results.
An engine operating state monitoring means comprises an engine speed sensor, a cooling water sensor, a throttle valve opening sensor and the like. An intake airflow measuring device is used to calculate a basic fuel injection quantity.
A typical example of the intake airflow measuring device is the heat wire type airflow measuring device disclosed in Japanese Patent Disclosure No. 55-98621. This device utilizes heat dissipation effect of airflow. It has a heater with a temperature-resistance characteristic providing a resistance corresponding to a temperature, which is arranged in the intake manifold. Heating power is supplied to the heater to monitor changes in temperatures. More specifically, terminal voltage at the heater is compared with a reference voltage. The heating power supplied to the heater is fed back to the heater such that the heater temperature is kept equal to the specific temperature.
Since the heating power supplied to the heater is controlled such that the heater temperature is kept equal to the predetermined temperature, the output changes only by a factor of 2 even if the airflow changes to a value 100 times the original value. Therefore, the measuring sensitivity of the intake airflow is very low.
In order to supply intake airflow measurement data to the electronic engine control unit from such an intake airflow measuring device to achieve proper engine control, an offset processor must be added to an amplifier for amplifying the airflow detection signal, resulting in a complex circuit arrangement.
When an engine control unit includes a microcomputer, the output signal from the airflow measuring device must be a digital signal. When the airflow measurement signal consists of analog data, e.g., a current, a high-performance A/D converter must be used to convert this analog data to digital data. When the heater temperature is maintained at a given temperature by supplying pulsed heating power in an intermittent manner, the intake airflow can be detected by the pulse duty. In this case, however, a complex signal processing means is required for processing and calculating the pulse duty.